Methods of charging lithium sulfur cells

ABSTRACT

Disclosed is a method of charging a lithium-sulfur electrochemical cell wherein the lithium-sulfur cell comprises a cathode comprising an electroactive sulfur-containing material, an anode comprising lithium, and a nonaqueous electrolyte.

TECHNICAL FIELD

The present invention relates generally to the field of electrochemicalcells and batteries, and to an electrolyte for use in an electrochemicalcell. More particularly, this invention pertains to electrochemicalcells where the cathode preferably comprises an electroactivesulfur-containing material and the anode preferably comprises lithium,and the cells deliver a high percentage of the theoretical dischargecapacity, exhibit high charge-discharge efficiency, and/or show lowself-discharge rates.

BACKGROUND

There has been considerable interest in recent years in developing highenergy density batteries with lithium-containing anodes. Lithium metalis particularly attractive as the anode active material ofelectrochemical cells because of its light weight and high energydensity, as compared, for example, to anode active materials such aslithium intercalated carbon anodes, where the presence ofnon-electroactive materials increases the weight and volume of theanode, thereby reducing the energy density of the anode. The use oflithium metal anodes, or those comprising lithium metal, provides anopportunity to construct cells that are lighter in weight and have ahigher energy density than cells such as lithium-ion, nickel metalhydride or nickel-cadmium cells. These features are highly desirable forbatteries in portable electronic devices such as cellular telephones andlaptop computers, as noted, for example, by Linden in Handbook ofBatteries, 1995, 2^(nd) Edition, chapter 14, pp. 75-76, and chapter 36,p.2, McGraw-Hill, New York, and in U.S. Pat. No. 6,406,815 to Sandberget al., the respective disclosures of which are incorporated herein byreference.

Thin film battery design is particularly suitable for portableelectronic devices in that it brings light weight, and the high surfacearea allows high rate capability as well as reduced current density oncharging. Several types of cathode materials for the manufacture ofthin-film lithium batteries are known, and include cathode materialscomprising sulfur-sulfur bonds, wherein high energy capacity andrechargeability are achieved from the electrochemical cleavage (viareduction) and reformation (via oxidation) of sulfur-sulfur bonds.Sulfur containing cathode materials, having sulfur-sulfur bonds, for usein electrochemical cells having lithium or sodium anodes includeelemental sulfur, organo-sulfur, or carbon-sulfur compositions.

For rechargeable lithium/sulfur (Li/S) cells there is a need for furtherenhancement of cell performance. Ideally cells should have highutilization at practical discharge rates over many cycles. Completedischarge of a cell over time periods ranging from 20 minutes (3 C) to 3hours (C/3) is typically considered a practical discharge rate. A numberof approaches have been explored for the improvement of the performanceand properties, such as utilization, self-discharge, charge-dischargeefficiency, and overcharge protection.

Lithium anodes in nonaqueous electrochemical cells develop surface filmsfrom reaction with cell components including solvents of the electrolytesystem and materials dissolved in the solvents, such as, for example,electrolyte salts and materials that enter the electrolyte from thecathode. Materials entering the electrolyte from the cathode may includecomponents of the cathode formulations and reduction products of thecathode formed upon cell discharge. In electrochemical cells withcathodes comprising sulfur-containing materials reduction products mayinclude sulfides and polysulfides. The composition and properties ofsurface films on lithium electrodes have been extensively studied, andsome of these studies have been summarized by Aurbach in NonaqueousElectrochemistry, Chapter 6, pages 289-366, Marcel Dekker, New York,1999. The surface films have been termed solid electrolyte interface(SEI) by Peled, in J. Electrochem. Soc., 1979, vol. 126, pages2047-2051.

The SEI may have desirable or undesirable effects on the functioning ofan electrochemical cell, depending upon the composition of the SEI.Desirable properties of an SEI in an electrochemical cell comprising alithium anode include being conductive to lithium ions and at the sametime preventing or minimizing lithium consuming reactions, such as thosewith electrolyte salts, electrolyte solvents, or soluble cathodereduction (discharge) products. Undesirable properties of the SEI mayinclude reduced discharge voltage and reduced capacity of the cell.Soluble cathode reduction products from sulfur-containing cathodematerials are known to be very reactive toward lithium anodes indicatingthat any SEI formed in Li/S cells is typically ineffective in preventingor minimizing lithium consuming reactions (these reactions are oftentermed lithium corrosion).

Approaches to protect lithium in Li/S cells have been described by Viscoet al. in U.S. Pat. No. 6,025,094; by Nimon et al. in U.S. Pat. Nos.6,017,651 and 6,225,002; and by Skotheim et al. in U.S. patentapplication Ser. Nos. 09/721,578 and 09/864,890.

Sulfur utilization in Li/S cells is dependent on a number of factors,including among others, formulation of the cathode, discharge rate,temperature, and electrolyte composition. As used herein, “a 100%utilization” (also called “sulfur utilization”) assumes that if allelemental sulfur in an electrode is fully utilized, the electrode willproduce 1675 mAh per gram of sulfur initially present in the electrode.Among the prior art references that discuss and teach sulfur utilizationare the following:

(1) U.S. Pat. No. 4,410,609 Peled et al. claimed to have achieved sulfurutilization of about 90% in Li/S cells employing THF or THF/tolueneelectrolyte solvents, but only at very low discharge rates (two monthsfor a single discharge).

(2) Peled et al. in J. Electrochem. Soc., 1989, vol. 136, pp. 1621-1625found that in dioxolane solvent mixtures similar Li/S cells achieve asulfur utilization of no more than 50% at discharge rates of 0.1 mA/cm²and 0.01 mA/cm².

(3) Chu in U.S. Pat. No. 5,686,201 describes a Li/S cell with apolymeric electrolyte that delivers 54% utilization at 30 ° C. and a lowdischarge rate of 0.02 mA/cm². At 90 ° C. a utilization of 90% at adischarge rate of 0.1 mA/cm² was achieved.

(4) Chu et al. in U.S. Pat. No. 6,030,720 describe liquid electrolyteLi/S rechargeable cells with sulfur utilization of approximately 40% formore than 70 cycles at discharge rates of 0.09 mA/cm² (90 μA/cm²) and0.5 mA/cm² (500 μA/cm²). Another example (Example 4) describes a sulfurutilization of 60% over more than 35 cycles but at the low dischargerate of 0.09 mA/cm².

(5) Cheon et al. in J. Electrochem. Soc., 2003, vol. 150, pp. A800-A805,describe various properties including rate capability and cyclecharacteristics of rechargeable Li/S cells. In FIG. 5 are shown chargeand discharge profiles for Li/S cells, with 0.5 M lithium triflate intetraglyme as electrolyte, from which charge-discharge efficiencies canbe estimated. A charge-discharge efficiency at the 30^(th) cycle ofapproximately 67% is estimated for cells charged at 0.26 mA/cm² anddischarged at 0.26 mA/cm² and an efficiency of approximately 48% forcells charged at 0.26 mA/cm² and discharged at 1.74 mA/cm². The sulfurutilization of these same cells is shown to be 37% and 28%,respectively, at the 30^(th) cycle.

Many lithium-based electrochemical cells, including Li/S cells, may berecharged by applying external current to the cell. The generalmechanism for recharging many lithium rechargeable cells is described byHossain in Handbook of Batteries, 1995, 2^(nd) Edition, chapter 36,p.1-28, McGraw-Hill, New York, and for Li/S cells by Mikhaylik et al. inJ. Electrochem. Soc., 2003, vol. 150, pp. A306-A311. When a cell isrecharged it may be unintentionally overcharged, which could lead tovarious undesirable reactions such as destruction of the cellelectrolyte, corrosion of the current collectors, degradation of thecell separators, and irreversible damage to the positive or negativeelectrode. Overcharge protection has been provided in lithium cells bythe use of redox shuttle additives, as described, for example, byNarayanan et al. in J. Electrochem. Soc., 1991, vol. 138, pp. 2224-2229,Golovin et al. in J. Electrochem. Soc., 1992, vol.139, pp.5-10, andRichardson et al. in J. Electrochem. Soc., 1996, vol. 143, pp.3992-3996. The redox shuttle additive species is oxidized at the cathodeduring overcharge, diffuses to the anode electrode, where it is reducedto the original species and diffuses back.

In Li/S cells an intrinsic redox shuttle is known that providesovercharge tolerance or protection, as described, for example, in U.S.Pat. No. 5,686,201 to Chu. Chu et al. in Proceedings of the 12^(th)Annual Battery Conference on Applications & Advances, 1997, pp. 133-134,state that the shuttle in Li/S cells limits overcharging, and provideexamples of cell voltage holding constant during extended overcharge asillustrated in FIG. 4 on page 134.

U.S. Pat. No. 5,882,812 to Visco et al. describes a method of protectionof rechargeable electrochemical energy conversion devices against damagefrom overcharge. Specifically, such devices may be characterized asincluding the following elements: (1) a negative electrode; (2) apositive electrode containing one or more intermediate species that areoxidized to one or more oxidized species during overcharge; and (3) atuning species that adjusts the rate at which the oxidized species arereduced, thereby adjusting the voltage at which overcharge protection isprovided. The oxidized species produced during overcharge move to thenegative electrode where they are reduced back to the intermediatespecies as in a normal redox shuttle. The overcharge protection systemsare described as applicable to many different cells, particularly thosewith alkali metal negative electrodes, including lithium/organosulfurcells, lithium/(inorganic sulfur) cells, lithium/(metal oxide) cells,lithium/(metal sulfide) cells, and carbon anode cells. The tuningspecies described include organosulfur compounds, and surface activeagents including: organoborates such as trimethylborate, boroxines, suchas trimethylboroxine; phosphorus containing compounds includingpolyphosphazenes and phosphates such as Li₃PO₄; carbonates such asLi₂CO₃; nitrogen containing compounds including nitrates such as LiNO₃;and organonitrogen compounds such as phenylhydrazine.

Gan et al. in U.S. Pat. Nos. 6,136,477 and 6,210,839 describe nitratesand nitrites as additives for electrolytes in lithium ion cells toreduce 1^(st) cycle irreversible capacity. In U.S. Pat. No. 6,060,184Gan et al. describe nitrate additives for nonaqueous electrolytes thatprovide increased discharge voltage and reduced voltage delay in currentpulse discharge, for example in alkali metal cells with SVO(silver-vanadium oxide) positive electrodes.

Redox shuttles in electrochemical cells, however, have also been shownto have an undesirable impact on cell properties, such as leading toself-discharge. Rao et al. in J. Electrochem. Soc., 1981, vol. 128, pp.942-945, the disclosure of which is incorporated herein by reference,describe the self discharge of Li/TiS₂ cells due to the presence ofelemental sulfur impurities, which act through a redox shuttlemechanism. The sulfur impurities become part of a polysulfide shuttle.Sulfide ions or low chain polysulfides are oxidized at the cathode tohigher polysulfides that are soluble in the electrolyte. These higherpolysulfides diffuse through the electrolyte to the anode where they arereduced to lower polysulfides that in turn diffuse back through theelectrolyte to the cathode to be reoxidized to higher polysulfides. Thisredox shuttle causes a continuous current flow in the cell, resulting ina depletion of the cell's stored capacity. This phenomenon is calledself-discharge. In U.S. Pat. No. 4,816,358 Holleck et al. describe amethod of reducing self-discharge in lithium cells, such as Li/TiS₂cells, which comprise cathodes containing sulfur as an impurity. Themethod uses scavengers, for example, metals or metal ions, that reactwith sulfur impurities to form stable sulfides thus reducing selfdischarge.

For rechargeable batteries, determining the point at which to terminatecharging is important for efficient charging, longevity of the battery,and for safety. A number of methods are known for charging batteries andfor determining the point of termination of the charge. U.S. Pat. No.5,900,718 to Tsenter and U.S. Pat. No. 5,352,967 to Nutz et al.summarize some of these charging and charge termination methodsparticularly useful for nickel batteries, such as nickel-cadmium,nickel-hydrogen and nickel metal-hydride. Prominent among thetermination methods are delta temperature/delta time (dT/dt), deltavoltage/delta time (dV/dt), and termination at a predetermined voltage.

SUMMARY OF THE INVENTION

The present invention provides electrolytes for lithium/sulfurelectrochemical cells that exhibit at least one of (1) lowself-discharge rates, (2) high cathode utilization rates, (3) highcharge-discharge efficiencies, and/or (4) high specific capacity. Theelectrolyte compositions comprise one or more N—O compounds inconcentrations from about 0.02 m to about 2.0 m. Surprisingly, theexample embodiments of Li/S electrochemical cells comprising the N—Oelectrolyte additives of the present invention show low rates ofreaction of cell components with lithium metal of the anode, improveddischarge capacity, and high charge-discharge efficiency.

The invention also comprises electrochemical cells and batteriesincluding the electrolyte, batteries that have improved charge/dischargeefficiencies, and a method of charging a cell or battery including theelectrolyte and/or having improved charge/discharge efficiencies.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the discharge of a cell of Example 4 (□) versus acell of Comparative Example 1 (no additive) (●).

FIG. 2 illustrates discharge of the cell of Example 24, that shows 100%sulfur utilization.

FIG. 3 illustrates the open cell voltage (OCV) for a cell with additiveLiNO₃ (B) (Example 16) and without additive (A).

FIG. 4 illustrates charge-discharge efficiency vs. cycle number: A for acell of Comparative Example 1 (no additive); B for a cell of Example 1;C for a cell of Example 12; D for a cell of Example 4; and E for a cellof Example 5.

FIG. 5 illustrates the charge profile at the 5^(th) charge cycle: A fora cell of Comparative Example 1 (no additive); B for a cell of Example 1(0.002 m LiNO₃); C for a cell of Example 2 (0.1 m LiNO₃); D for a cellof Example 3 (0.2 m LiNO₃); E for a cell of Example 4 (0.4 m LiNO₃); andF for a cell of Example 5 (1.55 m LiNO₃).

DETAILED DESCRIPTION OF THE INVENTION

One aspect of the present invention provides an electrochemical cellcomprising: (a) an anode comprising lithium; (b) a cathode comprising anelectroactive sulfur-containing material; and (c) a nonaqueouselectrolyte; wherein the electrolyte comprises: (i) one or morenonaqueous solvents; and (ii) one or more N—O additives.

Liquid electrolyte lithium/sulfur cells typically comprise an anodecomprising lithium, a cathode comprising an electroactivesulfur-containing material, a nonaqueous electrolyte, and a separatorinterposed between the anode and cathode, as described, for example, inU.S. Pat. No. 6,210,831 to Gorkovenko et al. and U.S. Pat. No. 5,919,587to Mukherjee et al., the respective disclosures of which areincorporated herein by reference. Following are descriptions of thepreferred anode, cathode, separator, electrolyte, and electrolyteadditive of an electrochemical cell according to the invention.

Anode

The anode may be of any structure suitable for use in a givenelectrochemical cell and with a given cathode. Suitable anode activematerials, comprising lithium, for the anodes of the present inventioninclude, but are not limited to, lithium metal, such as lithium foil andlithium deposited onto a substrate, such as a plastic film, and lithiumalloys, such as lithium-aluminum alloys and lithium-tin alloys. Lithiumanodes comprising multi-layer coatings such as those described in U.S.patent application Ser. No. 09/721,578 to Skotheim et al., thedisclosure of which that describes lithium anodes is incorporated hereinby reference, may also be used.

Cathode Active Layers

The cathode of a cell according to the present invention comprisescathode active layers including an electroactive sulfur-containingmaterial. The preferred cathode active layers are coated ontosubstrates, such as current collectors, to form composite cathodes,although any cathode structure that includes electroactivesulfur-containing material may be used. The term “electroactivesulfur-containing material,” as used herein, relates to cathode activematerials which comprise the element sulfur in any form, wherein theelectrochemical activity involves the breaking or forming ofsulfur-sulfur covalent bonds. Examples of suitable electroactivesulfur-containing materials include, but are not limited to, elementalsulfur and organic materials comprising both sulfur atoms and carbonatoms, which may or may not be polymeric. Suitable organic materialsinclude those further comprising heteroatoms, conductive polymersegments, composites, and conductive polymers.

In one embodiment, the electroactive sulfur-containing materialcomprises elemental sulfur. In another embodiment, the electroactivesulfur-containing material comprises a mixture of elemental sulfur and asulfur-containing polymer.

Suitable sulfur-containing organic polymers include, but are not limitedto, those described in U.S. Pat. Nos. 5,601,947; 5,690,702; 5,529,860;and 6,117,590 to Skotheim et al.; and U.S. Pat. No. 6,201,100 toGorkovenko et al.; all of a common assignee, and are incorporated hereinby reference in their entirety.

The electroactive sulfur-containing cathodes of the present inventionmay further comprise electroactive metal chalcogenides, electroactiveconductive polymers, and combinations thereof, for example, as describedin U.S. Pat. No. 5,919,587 to Mukherjee et al. and U.S. Pat. No.6,201,100 to Gorkovenko et al., the respective disclosures of which thatdescribe sulfur-containing cathodes are incorporated herein byreference.

The cathode active layers may further comprise one or more conductivefillers to provide enhanced electronic conductivity, for example, asdescribed in U.S. Pat. No. 6,194,099 to Geronov et al. and U.S. Pat. No.6,210,831 to Gorkovenko et al., the disclosures of which that describesulfur-containing cathodes are incorporated herein by reference. Thecathode active layers may also comprise a binder. The choice of bindermaterial may vary widely. Useful binders are those materials, usuallypolymeric, that allow for ease of processing of battery electrodecomposites and are known to those skilled in the art of electrodefabrication.

The cathode active layers may further comprise one or more N—O additiveof the present invention.

Separator

The electrochemical cells of the present invention may further comprisea separator interposed between the cathode and anode, although aseparator is optional. Typically, the separator is a porousnon-conductive or insulative material which separates or insulates theanode and the cathode from each other and which permits the transport ofions through the separator between the anode and the cathode.

A variety of separator materials are known in the art. Examples ofsuitable solid porous separator materials include, but are not limitedto, polyolefins, such as, for example, polyethylenes and polypropylenes,glass fiber filter papers, and ceramic materials. Further examples ofseparators and separator materials suitable for use in this inventionare those comprising a microporous pseudo-boehmite layer, which may beprovided either as a free standing film or by a direct coatingapplication on one of the electrodes, as described in U.S. Pat. No.6,153,337, by Carlson et al., the disclosure of which related to thestructure of separators and separator materials is incorporated hereinby reference. The additive of the present invention may be added to theseparator during cell assembly or incorporated in a coating process.Separators of a wide range of thickness may be used, for example fromabout 5 μm to about 50 μm, preferably from about 5 μm to about 25 μm.

Nonaqueous Electrolyte

The electrolytes used in electrochemical cells function as a medium forthe storage and transport of ions, and in the case of solid electrolytesand gel electrolytes, these materials may additionally function asseparator materials between the anode and the cathode. Any liquid,solid, or gel material capable of storing and transporting ions may beused as an electrolyte in the invention, so long as the material issubstantially electrochemically and chemically unreactive with respectto the anode and the cathode, and the material facilitates the transportof lithium ions between the anode and the cathode. The electrolyte mustalso be electronically non-conductive to prevent short circuitingbetween the anode and the cathode.

Typically, the electrolyte comprises one or more ionic electrolyte saltsto provide ionic conductivity and one or more nonaqueous liquidelectrolyte solvents, gel polymer materials, or solid polymer materials.Suitable nonaqueous electrolytes for use in the present inventioninclude, but are not limited to, organic electrolytes comprising one ormore materials selected from the group consisting of liquidelectrolytes, gel polymer electrolytes, and solid polymer electrolytes.Examples of nonaqueous electrolytes for lithium batteries are describedby Dominey in Lithium Batteries, New Materials, Developments andPerspectives, Chapter 4, pp. 137-165, Elsevier, Amsterdam (1994) andexamples of gel polymer electrolytes and solid polymer electrolytes aredescribed by Alamgir et al. in Lithium Batteries, New Materials,Developments and Perspectives, Chapter 3, pp. 93-136, Elsevier,Amsterdam (1994), the respective disclosures of which are incorporatedherein by reference.

Organic solvents for use in a nonaqueous electrolyte according to theinvention include, but are not limited to, families such as, acetals,ketals, sulfones, acyclic ethers, cyclic ethers, glymes, polyethers,dioxolanes, substituted forms of the foregoing, and blends thereof.

Examples of acyclic ethers that may be used include, but are not limitedto, diethyl ether, dipropyl ether, dibutyl ether, dimethoxymethane,trimethoxymethane, dimethoxyethane, diethoxyethane,1,2-dimethoxypropane, and 1,3-dimethoxypropane.

Examples of cyclic ethers that may be used include, but are not limitedto, tetrahydrofuran, tetrahydropyran, 2-methyltetrahydrofuran,1,4-dioxane, 1,3-dioxolane, and trioxane.

Examples of polyethers that may be used include, but are not limited to,diethylene glycol dimethyl ether (diglyme), triethylene glycol dimethylether (triglyme), tetraethylene glycol dimethyl ether (tetraglyme),higher glymes, ethylene glycol divinylether, diethylene glycoldivinylether, triethylene glycol divinylether, dipropylene glycoldimethyl ether, and butylene glycol ethers.

Examples of sulfones that may be used include, but are not limited to,sulfolane, 3-methyl sulfolane, and 3-sulfolene.

The specific choice of solvent for a particular cell will depend on oneor more of several factors, including the composition of the anode andcathode, and the solubility of the lithium salts of the anions generatedduring discharge of the electroactive sulfur-containing material of thecathode. Although a single solvent may be used, a solvent mixturecomprising two or more solvents selected from acyclic ethers, glymes andrelated polyethers, and cyclic ethers, such as 1,3-dioxolane ispreferred. Preferred mixtures of solvents include, but are not limitedto, 1,3-dioxolane and dimethoxyethane, 1,3-dioxolane anddiethyleneglycol dimethyl ether, 1,3-dioxolane and triethyleneglycoldimethyl ether, and 1,3-dioxolane and sulfolane. The weight ratio of thetwo solvents in the preferred binary mixtures may vary from about 5 to95 to 95 to 5. Preferred are mixtures comprising dioxolane. Mostpreferred are mixtures comprising greater than 40% by weight ofdioxolane.

Ionic electrolyte lithium salts may be added to the electrolyte toincrease the ionic conductivity. Examples of ionic electrolyte lithiumsalts for use in the present invention include, but are not limited toone or more of, LiSCN, LiBr, LiI, LiClO₄, LiAsF₆, LiSO₃CF₃, LiSO₃CH₃,LiBF₄, LiB(Ph)₄, LiPF₆, LiC(SO₂CF₃)₃, and LiN(SO₂CF₃)₂. Preferred ionicelectrolyte lithium salts are LiSCN, LiSO₃CF₃, and LiN(SO₂CF₃)₂. A rangeof concentrations of the ionic lithium salts in the solvent may be usedsuch as from about 0.2 m to about 2.0 m (m is moles/kg of solvent).Preferred concentrations are from about 0.5 m to about 1.5 m. Theaddition of ionic lithium salts to the solvent is optional in that upondischarge of Li/S cells the lithium sulfides or polysulfides formedtypically provide ionic conductivity to the electrolyte, which may makethe addition of ionic lithium salts unnecessary.

Furthermore, if the ionic N—O additive of the present invention is, forexample, inorganic nitrate, organic nitrate, or inorganic nitrite it mayprovide ionic conductivity to the electrolyte in which case noadditional ionic lithium electrolyte salts may be needed.

Additive

N—O compounds for use as additives in the electrolyte of the presentinvention include, but are not limited to, families such as inorganicnitrates, organic nitrates, inorganic nitrites, organic nitrites,organic nitro compounds, and other organic N—O compounds.

Examples of inorganic nitrates that may be used include, but are notlimited to, lithium nitrate, potassium nitrate, cesium nitrate, bariumnitrate, and ammonium nitrate.

Examples of organic nitrates that may be used include, but are notlimited to, dialkyl imidazolium nitrates, and guanidine nitrate.

Examples of inorganic nitrites that may be used include, but are notlimited to, lithium nitrite, potassium nitrite, cesium nitrite, andammonium nitrite.

Examples of organic nitrites that may be used include, but are notlimited to, ethyl nitrite, propyl nitrite, butyl nitrite, pentylnitrite, and octyl nitrite.

Examples organic nitro compounds that may be used include, but are notlimited to, nitromethane, nitropropane, nitrobutanes, nitrobenzene,dinitrobenzene, nitrotoluene, dinitrotoluene, nitropyridine, anddinitropyridine.

Examples of other organic N—O compounds that may be used include, butare not limited to, pyridine N-oxide, alkylpyridine N-oxides, andtetramethyl piperidine N-oxyl (TEMPO).

Concentrations of the N—O additive in the electrolytes are from about0.02 m to about 2.0 m. Preferred concentrations are from about 0.1 m toabout 1.5 m. The most preferred concentrations are from 0.2 m to 1.0 m.Concentrations of the ionic N—O additive when used in embodiments thatdo not include added lithium salts may vary from about 0.2 m to about2.0 m.

Although it is preferred to incorporate the N—O additive into theelectrolyte that is added to the lithium/sulfur cells duringfabrication, the N—O additive may first be introduced into the cell as apart of other cell components from where it can enter the electrolyte.The N—O additive may be incorporated into liquid, gel or solid polymerelectrolytes. The N—O additive may be incorporated in the cathodeformulation or into the separator in the fabrication process, as long asit is included in a manner such that it will enter the electrolyte insufficient concentrations. Thus during discharge and charge of the cellthe N—O additive incorporated in the cathode formulation or theseparator would dissolve in the electrolyte.

Utilization

As used herein, “utilization” assumes that if all elemental sulfur in anelectrode is fully utilized, the electrode will produce 1675 mAh/g ofsulfur. That is, 100% utilization corresponds to 1675 mAh/g of thesulfur in the cell, 90% utilization corresponds to 1507.5 mAh/g, 60%utilization corresponds to 1005 mAh/g, and 50% utilization correspondsto 837.5 mAh/g of sulfur in the cell.

Sulfur utilization varies with the discharge current applied to thecell, among other things. Sulfur utilization at low discharge rates ishigher than at high discharge rates, for example, as described forprimary cells in U.S. Pat. No. 4,410,609 to Peled et al. and asdescribed for secondary cells in U.S. Pat. No. 6,030,720 to Chu et al.and by Cheon et al. in J. Electrochem. Soc., 2003, vol. 150, pp.A800-A805.

Typically, secondary cells of this invention will cycle at least 10times, preferably at least 20 times and more preferably at least 50times, with each cycle having a sulfur utilization of at least about 60%when discharged at a moderately high discharge current of 340 mA/g ofsulfur (0.41 mA/cm² for the 846 cm² cells of the Examples). Thisdischarge rate results in a preferred discharge period of less than 4hours for the cells of the present invention, such as for example, thecells of Examples 2-12. As illustrated in Example 27 and Table 4 cellsof the present invention deliver sulfur utilization in excess of 65% atvery high discharge currents up to 3600 mA (3495 mA/g) (equivalent to4.26 mA/cm²). At this discharge current these cells are fully dischargedin less than 20 minutes (a 3 C rate).

The additive of the present invention increases utilization by about 20%or more in the tests set forth in the Examples. Typically the increasein utilization is from about 20% to more than 35% dependent upondischarge rate, N—O additive composition, and N—O additiveconcentration. For example, the discharge capacity at the 5^(th) cycleof the cells of Example 2 show an increase in utilization of 36%compared with the cells of Comparative Example 1 (1226 mAh/g vs. 901mAh/g). The cells of Example 9, Example 13, and Example 19 showincreases in utilization of 24%, 23%, and 32%, respectively, comparedwith the cells of Comparative Example 1.

The additive of the present invention enhances sulfur utilization over arange of additive concentrations. Although low concentrations, forexample less than 0.1 m, can be used, enhanced sulfur utilization mayonly be obtained for a limited number of cycles at low concentrations.

Self-Discharge

It is highly desirable that batteries retain their capacity duringprolonged storage under ambient conditions. However, battery storagetypically leads to a loss of charge retention, often termedself-discharge. “Self-discharge,” as used herein, pertains to thedifference between the discharge capacity of a cell at the N^(th) cycleand the discharge capacity at the (N+1)^(th) cycle after a storageperiod in a charged state:${{{Self}\text{-}{discharge}\quad(\%)} = {\frac{C^{N} - C^{N + 1}}{C^{N}} \times 100\quad\%}},$

-   -   where C^(N) is the N^(th) cycle discharge capacity of the cell        (mAh) and C^(N+1) is the (N+1)^(th) cycle discharge capacity of        the cell (mAh) after a storage period.

Factors that influence charge retention, as summarized by Linden, inHandbook of Batteries, 2^(nd) Edition, pp. 3.18-3.19, McGraw Hill, NewYork, 1995, include, for example, storage conditions such astemperature, length of storage, cell design, the electrochemical system,and discharge conditions.

One approach to reducing self-discharge during storage in Li/S cells isdescribed in U.S. Pat. No. 6,436,583 to Mikhaylik in which theelectrolyte comprises one or more organic sulfites. The self-dischargeinhibiting organic sulfites are particularly effective in fresh cellsbut may be removed by reaction with polysulfides produced during celldischarge.

As summarized in Table 1, self-discharge of the examples set forthherein was determined by comparing discharge capacity at the 5^(th)discharge cycle (2 min after charge) with capacity at the 6^(th)discharge cycle measured after storage in the fully charged state for 24hours at approximately 25° C. For the cells of Comparative Example 1without additive the self-discharge is (901−775)/901×100%=14%. For thecells of Example 4 the self-discharge is (1155−1109)/1155×100%=4%. Forthe cells of Example 13 the self-discharge is(1107−1023)/1107×100%=7.6%. Further, it can be seen from Table 1 that asa result of the lowered self-discharge due to inclusion of an N—Oadditive, sulfur utilization remained high after the cells of thepresent invention were stored. After storage, cells of Examples 4, 5, 6,and 8 showed a sulfur utilization of at least 60% whereas the cells ofComparative Example 1 showed sulfur utilization of only 46%.

Self discharge can also be monitored by measuring open cell voltage(OCV) of the fully charged cell during storage. OCV declines as capacityis lost due to self discharge. As illustrated in FIG. 3 cells of Example16 of the present invention show little change in OCV upon storage for30 days in the fully charged condition when stored after 34discharge-charge cycles, which indicates a very low rate ofself-discharge. As also illustrated in FIG. 3, cells without theadditive of the present invention show a rapid change in OCV in lessthan 20 hours when stored under the same conditions, which indicates ahigh rate of self-discharge.

Charge-Discharge Efficiency

The term “charge-discharge efficiency” as used herein, represents theratio of capacity obtained on discharge divided by the capacity suppliedin the prior charge step. In other words, charge-discharge efficiency,C_(eff)=D_(n+1)/C_(n)×100%, where D is discharge capacity, C is chargecapacity and n is the cycle number. The additive of the presentinvention increases the charge-discharge efficiency of Li/S cells. Forexample, the cells of Example 4 in which the additive is lithium nitrate(LiNO₃) show a charge-discharge efficiency of 98.8% whereas the cells ofComparative Example 1 without additive show a charge-dischargeefficiency of only 66.3%, as shown in Table 1 (both measured at the4^(th) charge cycle and the 5^(th) discharge cycle). The highcharge-discharge efficiency is maintained during further cycling asillustrated in FIG. 4.

While not being bound by any specific theory, it is believed that thesuperior charge-discharge efficiency results are achieved for, amongother things, the following reasons. In Li/S cells during the chargingprocess sulfide ions or low chain polysulfides are oxidized at thecathode to higher polysulfides, which are soluble in the electrolyte.These higher polysulfides diffuse to the anode where they are reduced tolower polysulfides, which in turn diffuse back to the cathode to bereoxidized. This redox shuttle causes a continuous current flow in thecell, resulting in a reduction of the cell's storage capacity and alowered charge-discharge efficiency. A similar redox process occursduring self-discharge. The additive of the present invention, it isbelieved, essentially inactivates the shuttle in Li/S cells, whichresults in much higher charge-discharge efficiencies.

Charge Termination by Voltage

In the charging process of rechargeable cells it is important to be abledetermine when the cell is fully charged because overcharging isdamaging to a cell as well as time wasting. Cells may show a sharpchanges in temperature or voltage at the point of reaching full charge.For example, at the end of charge lithium ion cells show a sharpincrease in voltage, as described, by Golovin et al. in J. Electrochem.Soc., 1992, vol.139, pp.5-10. In contrast, as illustrated in FIG. 5A,the Li/S cells of Comparative Example 1, lacking the N—O additive,exhibit a voltage profile which reaches a plateau at about 2.3 volts anddoes not increase with prolonged charging. This curve resembles theshape of the voltage curve for a lithium ion cell under charge to whichhas been added a redox shuttle additive, for example, as described byGolovin et al. As illustrated in FIG. 5C-F the cells of the presentinvention (Examples 2, 3, 4, and 5) comprising N—O additive exhibit avoltage profile upon charge at constant current that shows a sharpincrease in voltage as the cell reaches full capacity. The rapidincrease in voltage as the cell reaches full capacity in the cells ofthe present invention comprising N—O additive may be used to terminatethe charging process. For example, at a predetermined voltage withinthis region of rapid increase in voltage the charging process can beterminated.

In one method of the present invention, a Li/S cell is charged by (a)supplying electric energy at constant current; (b) monitoring voltageduring the charging; and (c) terminating the charge when the monitoredvoltage is in the range of about 2.35 volts to about 2.7 volts. In oneembodiment of the method, the charge is terminated in the range of about2.4 volts to about 2.6 volts. In a further embodiment of the method, thecharge is terminated at a voltage of about 2.5 volts. Typically chargingis performed by supplying constant current so as to charge the cell inabout 1 to 6 hours. For the cells of the Examples the currents are fromabout 200 mA to about 1200 mA, or about 0.24 mA/cm² to about 1.44mA/cm². The supply of constant current is typically provided with anaccuracy in the range of 1-5%; i.e. the current may vary ±1-5%. Voltageis typically monitored in the monitoring step at intervals varying fromabout 10 seconds to less than 1 second, depending among other things,for example, on the magnitude of the current and the length of charge.In an alternative charge termination method, a cell is charged atconstant current to a predetermined voltage; charging continued at thisvoltage until the charge current density falls to a value in the rangeof about 0.025 mA/cm² to about 0.01 mA/cm². In one method of the presentinvention, a Li/S cell is charged by (a) supplying electric energy atconstant current; (b) monitoring voltage during the charging; (c)supplying electric energy at constant current until the monitoredvoltage is about 2.5 volts; (d) holding the cell voltage at about 2.5volts while monitoring the charge current density; and (e) terminatingthe charge when the charge current density becomes less than 0.025mA/cm². In another embodiment of the method, the charge is terminated ata current density of less than 0.012 mA/cm². A current density of 0.025mA/cm² is equivalent to a current of 21 mA/g of sulfur and a currentdensity 0.012 mA/cm² is equivalent to a current of 10 mA/g of sulfur inthe cells of the Examples.

Although the use of voltage to determine the charge cutoff is preferredfor charge termination, a delta voltage/delta time (dV/dt) may also beused. For example, as the charging proceeds dV/dt rapidly increases atfull charge, and this point of rapid increase can used with appropriateelectronics for charge termination. As illustrated in Example 28 themagnitude of dV/dt increases by a factor more than 8 at about 2.5 V andby a further order of magnitude above 2.5 V. In another method of thepresent invention, a Li/S cell is charged by (a) supplying electricenergy at constant current; (b) monitoring voltage during the charging;(c) calculating the rate of change of voltage with time (dV/dt); and (d)terminating the charge when the value of dV/dt increases by more than 5times. In another embodiment the charge is terminated when the value ofdV/dt increases by more than 10 times. With the flat voltage profile ofLi/S cells lacking the N—O additive, overcharge of the cells invariablyoccurs and, furthermore, more complex charge termination methods arerequired which may be less reliable, less efficient, much less precise,may damage the cells, and be more costly.

The additive of the present invention is effective in providing a chargeprofile with a sharp increase in voltage at the point of full chargeover a range of concentrations from about 0.1 m to about 2.0 m. In oneembodiment, the concentration of the additive of the present inventionis from about 0.1 m to about 1.0 m. In a preferred embodiment, theconcentration of the additive of the present invention is from 0.1 m to0.5 m. Although low concentrations, for example less than 0.1 m, can beused, a charge profile with a sharp increase in voltage at the point offull charge may only be obtained for a limited number of cycles with lowconcentrations.

While not being bound by any specific theory, it is believed that theadditive of the present invention essentially inactivates (turns off)the shuttle in Li/S cells, which eliminates the flat voltage chargeprofile of Li/S cells without the additive, and substitutes a sharpvoltage increase at the point of full charge.

Cells and Batteries

Cells of the present invention may be made in a variety of sizes andconfigurations in any suitable fashion which are known to those skilledin the art. These battery design configurations include, but are notlimited to, planar, prismatic, jelly roll, w-fold, stacked and the like.Although the methods of the present invention are particularly suitablefor use with thin film electrodes, they may nevertheless be beneficialin thick film designs. Alternatively, designs incorporating both low andhigh surface area regions, as described in U.S. Pat. Nos. 5,935,724 and5,935,728 to Spillman et al., can be incorporated into jellyroll andother configurations.

Thin film electrodes may be configured into prismatic designs. With thedrive to conserve weight thin film barrier materials are particularlypreferred, e.g., foils. For example, U.S. Pat. No. 6,190,426 to Thibaultet al. describes methods for preparing prismatic cells in which suitablebarrier materials for sealed casing, methods of filling cells withelectrolyte, and methods of sealing the casing, are described, thedisclosure of which is incorporated herein by reference. When using thinfilm electrodes configured into prismatic designs it is important thatthe electrodes possess dimensional stability.

Batteries may be of any size or shape and may comprise one or more cellsaccording to the invention. For example, one or more of the prismaticcells described in U.S. Pat. No. 6,190,426 to Thibault et al. may beconnected to form a battery. Batteries comprising one or more cells maybe encased in a rigid casing, for example, as described in U.S. Pat. No.6,296,967 to Jacobs et al.

EXAMPLES

Several embodiments of the present invention are described in thefollowing examples, which are offered by way of illustration and not byway of limitation.

In the following Examples and Comparative Examples cells were preparedby the following method. The cathodes were prepared by coating a mixtureof 73 parts by weight of elemental sulfur, 22 parts by weight ofconductive carbon, and 5 parts by weight of a polyethylene powder,dispersed in isopropanol, onto a 6 micron thick conductive carbon coatedaluminum/polyester (PET) film substrate. After drying, the coatedcathode active layer thickness was about 28-29 microns. The anode waslithium foil of about 50 microns in thickness. The porous separator usedwas a 9 micron polyolefin separator. The above components were assembledin a layered structure of cathode/separator/anode, which was wound andcompressed, and placed in a foil pouch with liquid electrolyte(approximately 4.7 g). The prismatic cell had an electrode area of about846 cm². The sulfur content of the cell was 1.03 g, equivalent to 1725mAh capacity (1675 mAh/g×1.03 g). After sealing the cell in a foilpouch, it was stored for 24 hours and then re-sealed. Discharge-chargecycling of the cell was performed at 350 mA/200 mA, respectively, withdischarge cutoff at a voltage of 1.8 V and charge cutoff at 2.5 V (orfor 7 hrs if that was reached first). The discharge rate of 350 mA is0.414 mA/cm² for this cell (350 mA/846 cm²) and the charge rate of 200mA is 0.236 mA/cm² (200 mA/846 cm²). The pause after each charge anddischarge step was 2 minutes, unless otherwise noted. The temperaturefor the cell evaluation was between 22° C. and 25° C. The followingExamples and Comparative Examples describe the electrolytes evaluated inthese Li/S cells.

Comparative Example 1

The electrolyte was a 0.5 m solution of lithium bis(trifluoromethylsulfonyl) imide, (lithium imide) in a 50:50 weight ratiomixture of 1,3-dioxolane (DOL) and dimethoxyethane (DME). The dischargecapacity at the 5^(th) cycle was 928 mAh and specific capacity 901mAh/g. After the subsequent charge cycle (5^(th) charge cycle) the cellwas allowed to rest for 24 hours at ambient temperature (25° C.) beforedischarge (6^(th) discharge cycle). The discharge capacity at the 6^(th)cycle was 799 mAh and the specific capacity was 775 mAh/g of sulfur.Charge and discharge steps were resumed with the normal 2 minute pauseafter each. The discharge capacity at the 7^(th) cycle was 933 mAh andthe specific capacity was 906 mAh/g of sulfur. Charge-discharge cycleswere continued until the discharge capacity diminished to 900 mAh (874mAh/g of sulfur; 52% utilization), which was 15 cycles and theaccumulated capacity 14.1 Ah.

Example 1

The electrolyte was that of Comparative Example 1 except that lithiumnitrate at a concentration of 0.002 m was incorporated in the 0.5 melectrolyte solution of lithium imide in a 50/50 mixture of DOL and DME.In other words 0.002 moles of lithium nitrate (0.14 g) was added per Kgof the DOL/DME solvent (0.14 mg/g of solvent). Cycling of the cell wasperformed by the procedure of Comparative Example 1 with the resultsshown in Tables 2 and 3. Charge-discharge cycles were continued untilthe discharge capacity diminished to 900 mAh (874 mAh/g of sulfur; 52%utilization), which was 34 cycles and the accumulated capacity 33.7 Ah.9 cycles were achieved before utilization fell below 60% (1005 mAh/g ofsulfur).

Example 2

The electrolyte was that of Comparative Example 1 except that lithiumnitrate at a concentration of 0.1 m (6.9 mg/g of solvent) wasincorporated in the 0.5 m electrolyte solution of lithium imide in a50/50 mixture of DOL and DME. Cycling of the cell was performed by theprocedure of Comparative Example 1 with the results shown in Tables 2and 3. Charge-discharge cycles were continued until the dischargecapacity diminished to 900 mAh (874 mAh/g of sulfur), which was 33cycles and the accumulated capacity 37.1 Ah. 25 cycles were achievedbefore utilization fell below 60% (1005 mAh/g of sulfur).

Example 3

The electrolyte was that of Comparative Example 1 except that lithiumnitrate at a concentration of 0.2 m (13.8 mg/g of solvent) wasincorporated in the 0.5 m electrolyte solution of lithium imide in a50/50 mixture of DOL and DME. Cycling of the cell was performed by theprocedure of Comparative Example 1 with the results shown in Tables 2and 3. Charge-discharge cycles were continued until the dischargecapacity diminished to 900 mAh (874 mAh/g of sulfur; 52% utilization),which was 46 cycles and the accumulated capacity 51.6 Ah. 39 cycles wereachieved before utilization fell below 60% (1005 mAh/g of sulfur).

Example 4

The electrolyte was that of Comparative Example 1 except that lithiumnitrate at a concentration of 0.4 m (27.6 mg/g of solvent) wasincorporated in the 0.5 m electrolyte solution of lithium imide in a50/50 mixture of DOL and DME. Cycling of the cell was performed by theprocedure of Comparative Example 1 with the results shown in Tables 2and 3. Charge-discharge cycles were continued until the dischargecapacity diminished to 900 mAh (874 mAh/g of sulfur; 52% utilization),which was 63 cycles and the accumulated capacity 69.2 Ah. 50 cycles wereachieved before utilization fell below 60% (1005 mAh/g of sulfur).

Example 5

The electrolyte was that of Comparative Example 1 except that lithiumnitrate at a concentration of 1.55 m (107 mg/g of solvent) wasincorporated in the 0.5 m electrolyte solution of lithium imide in a50/50 mixture of DOL and DME. Cycling of the cell was performed by theprocedure of Comparative Example 1 with the results shown in Tables 2and 3. Charge-discharge cycles were continued until the dischargecapacity diminished to 900 mAh (874 mAh/g of sulfur; 52% utilization),which was 102 cycles and the accumulated capacity 105.8 Ah. 70 cycleswere achieved before utilization fell below 60% (1005 mAh/g of sulfur).

Example 6

The electrolyte was that of Comparative Example 1 except that theelectrolyte was made by the incorporation of lithium nitrate at aconcentration of 0.4 m (27.6 mg/g of solvent) in a 0.5 m solution oflithium trifluoromethyl sulfonate (lithium triflate) in a 50:50 weightratio mixture of DOL and DME. Cycling of the cell was performed by theprocedure of Comparative Example 1 with the results shown in Tables 2and 3. Charge-discharge cycles were continued until the dischargecapacity diminished to 900 mAh (874 mAh/g of sulfur; 52% utilization),which was 54 cycles and the accumulated capacity 56.6 Ah. 35 cycles wereachieved before utilization fell below 60% (1005 mAh/g of sulfur).

Example 7

The electrolyte was that of Comparative Example 1 except that potassiumnitrate at a concentration of approximately 0.1 m (10 mg/g of solvent)was incorporated in the 0.5 m electrolyte solution of lithium imide in a50/50 mixture of DOL and DME. Cycling of the cell was performed by theprocedure of Comparative Example 1 with the results shown in Tables 2and 3. Charge-discharge cycles were continued until the dischargecapacity diminished to 900 mAh (874 mAh/g of sulfur; 52% utilization),which was 28 cycles and the accumulated capacity 30.3 Ah. 26 cycles wereachieved before utilization fell below 60% (1005 mAh/g of sulfur).

Example 8

The electrolyte was that of Comparative Example 1 except that cesiumnitrate at a concentration of approximately 0.1 m (19 mg/g of solvent)was incorporated in the 0.5 m electrolyte solution of lithium imide in a50/50 mixture of DOL and DME. Cycling of the cell was performed by theprocedure of Comparative Example 1 with the results shown in Tables 2and 3. Charge-discharge cycles were continued until the dischargecapacity diminished to 900 mAh (874 mAh/g of sulfur; 52% utilization),which was 23 cycles and the accumulated capacity 24.8 Ah. 22 cycles wereachieved before utilization fell below 60% (1005 mAh/g of sulfur).

Example 9

The electrolyte was that of Comparative Example 1 except that ammoniumnitrate at a concentration of 0.013 m (1 mg/g of solvent) wasincorporated in the 0.5 m electrolyte solution of lithium imide in a50/50 mixture of DOL and DME. Cycling of the cell was performed by theprocedure of Comparative Example 1 with the results shown in Tables 2and 3. Charge-discharge cycles were continued until the dischargecapacity diminished to 900 mAh (874 mAh/g of sulfur; 52% utilization),which was 44 cycles and the accumulated capacity 45.3 Ah. 19 cycles wereachieved before utilization fell below 60% (1005 mAh/g of sulfur).

Example 10

The electrolyte was that of Comparative Example 1 except thatguanidinium nitrate at a concentration of 0.02 m (2.4 mg/g of solvent)was incorporated in the 0.5 m electrolyte solution of lithium imide in a50/50 mixture of DOL and DME. Cycling of the cell was performed by theprocedure of Comparative Example 1 with the results shown in Tables 2and 3. Charge-discharge cycles were continued until the dischargecapacity diminished to 900 mAh (874 mAh/g of sulfur; 52% utilization),which was 36 cycles and the accumulated capacity 35.5 Ah. 10 cycles wereachieved before utilization fell below 60% (1005 mAh/g of sulfur).

Example 11

The electrolyte was that of Comparative Example 1 except that potassiumnitrite (KNO₂) at a concentration of approximately 0.1 m (8 mg/g ofsolvent) was incorporated in the 0.5 m electrolyte solution of lithiumimide in a 50/50 mixture of DOL and DME. Cycling of the cell wasperformed by the procedure of Comparative Example 1 with the resultsshown in Tables 2 and 3. Charge-discharge cycles were continued untilthe discharge capacity diminished to 900 mAh (874 mAh/g of sulfur; 52%utilization), which was 17 cycles and the accumulated capacity 18.1 Ah.16 cycles were achieved before utilization fell below 60% (1005 mAh/g ofsulfur).

Example 12

The electrolyte was that of Comparative Example 1 except that3,4-dinitro-toluene at a concentration of 0.4 m (73 mg/g of solvent) wasincorporated in the 0.5 m electrolyte solution of lithium imide in a50/50 mixture of DOL and DME. Cycling of the cell was performed by theprocedure of Comparative Example 1 with the results shown in Tables 2and 3. Charge-discharge cycles were continued until the dischargecapacity diminished to 900 mAh (874 mAh/g of sulfur; 52% utilization),which was 39 cycles and the accumulated capacity 38.8 Ah. 13 cycles wereachieved before utilization fell below 60% (1005 mAh/g of sulfur).

Example 13

The electrolyte was that of Comparative Example 1 except thatnitromethane at a concentration of 0.4 m (24 mg/g of solvent) wasincorporated in the 0.5 m electrolyte solution of lithium imide in a50/50 mixture of DOL and DME. Cycling of the cell was performed by theprocedure of Comparative Example 1 with the results shown in Tables 2and 3. Charge-discharge cycles were continued until the dischargecapacity diminished to 900 mAh (874 mAh/g of sulfur; 52% utilization),which was 39 cycles and the accumulated capacity 41.6 Ah. 28 cycles wereachieved before utilization fell below 60% (1005 mAh/g of sulfur).

Example 14

The electrolyte was that of Comparative Example 1 except thatnitromethane at a concentration of 0.02 m (1.2 mg/g of solvent) wasincorporated in the 0.5 m electrolyte solution of lithium imide in a50/50 mixture of DOL and DME. Cycling of the cell was performed by theprocedure of Comparative Example 1 with the results shown in Tables 2and 3. Charge-discharge cycles were continued until the dischargecapacity diminished to 900 mAh (874 mAh/g of sulfur; 52% utilization),which was 23 cycles and the accumulated capacity 22.2 Ah. 1 cycle wasachieved before utilization fell below 60% (1005 mAh/g of sulfur).

Example 15

The electrolyte was that of Comparative Example 1 except that lithiumnitrate at a concentration of 0.5 m was incorporated in a DOL/DMEmixture (50/50 by weight) as the electrolyte solution, without lithiumimide. Cycling of the cell was performed by the procedure of ComparativeExample 1 with the results shown in Tables 2 and 3. Charge-dischargecycles were continued until the discharge capacity diminished to 900 mAh(874 mAh/g of sulfur; 52% utilization), which was 71 cycles and theaccumulated capacity 76.1 Ah. 47 cycles were achieved before utilizationfell below 60% (1005 mAh/g of sulfur).

Example 16

The electrolyte was that of Comparative Example 1 except that lithiumnitrate at a concentration of 1.24 m was incorporated in a 0.77 melectrolyte solution of lithium imide in a 50/50 mixture of DOL and DME.Cycling of the cell was performed by the procedure of ComparativeExample 1 for fifteen discharge and charge cycles. After the 15^(th)charge cycle the cell was stored at 25° C. for 8 days before the 16^(th)discharge cycle. The discharge capacity at the 15^(th) cycle was 1249mAh (specific capacity 1213 mAh/g of sulfur) and the discharge capacityat the 16^(th) cycle was 1195 mAh (specific capacity 1160 mAh/g ofsulfur). Sulfur utilization at the 15^(th) cycle was 72.4% and at the16^(th) cycle 69.3%. Self discharge during 8 days storage was(1249−1195)/1249×100%=4.3%. Cycling was resumed and after the 34^(th)charge cycle the cell was stored for 30 days. FIG. 3 shows the open cellvoltage during the 30 day storage period, showing very small voltagechange.

Example 17

The electrolyte was that of Comparative Example 1 except that1-nitropropane at a concentration of 0.4 m (35.6 mg/g of solvent) wasincorporated in the 0.5 m electrolyte solution of lithium imide in a50/50 mixture of DOL and DME. Cycling of the cell was performed by theprocedure of Comparative Example 1 with the results shown in Tables 2and 3. Charge-discharge cycles were continued until the dischargecapacity diminished to 900 mAh (874 mAh/g of sulfur; 52% utilization),which was 30 cycles and the accumulated capacity 30.2 Ah. 17 cycles wereachieved before utilization fell below 60% (1005 mAh/g of sulfur).

Example 18

The electrolyte was that of Comparative Example 1 except that TEMPO(tetramethyl piperidine N-oxyl) at a concentration of 0.09 m (14.0 mg/gof solvent) was incorporated in the 0.5 m electrolyte solution oflithium imide in a 50/50 mixture of DOL and DME. Cycling of the cell wasperformed by the procedure of Comparative Example 1 with the resultsshown in Tables 2 and 3. Charge-discharge cycles were continued untilthe discharge capacity diminished to 900 mAh (874 mAh/g of sulfur; 52%utilization), which was 19 cycles and the accumulated capacity 17.8 Ah.

Example 19

The electrolyte was that of Comparative Example 1 except that1-ethyl-3-methylimidazolium nitrate at a concentration of 0.12 m (20.8mg/g of solvent) was incorporated in the 0.5 m electrolyte solution oflithium imide in a 50/50 mixture of DOL and DME. Cycling of the cell wasperformed by the procedure of Comparative Example 1 with the resultsshown in Tables 2 and 3. Charge-discharge cycles were continued untilthe discharge capacity diminished to 900 mAh (874 mAh/g of sulfur; 52%utilization), which was 26 cycles and the accumulated capacity 29.4 Ah.25 cycles were achieved before utilization fell below 60% (1005 mAh/g ofsulfur). TABLE 1 Sulfur Utilization and Specific Capacity SpecificCapacity mAh/g Additive (Concentration (Sulfur Utilization) Examplemolal, m) 5^(th) Cycle 6^(th) Cycle* 7^(th) Cycle Comp. Ex. 1 None  901(54%)  75 (46%)  906 (54%) Example 1 LiNO₃ (0.002 m) 1065 (64%)  848(51%) 1021 (61%) Example 2 LiNO₃ (0.1 m) 1226 (73%)  934 (56%) 1215(73%) Example 3 LiNO₃ (0.2 m) 1196 (71%) 1015 (61%) 1200 (72%) Example 4LiNO₃ (0.4 m) 1155 (68%) 1109 (66%) 1158 (69%) Example 5 LiNO₃ (1.55 m)1106 (66%) 1035 (62%) 1102 (66%) Example 6 LiNO₃ (0.4 m) 1129 (67%) 1002(60%) 1077 (64%) Example 7 KNO₃ (<0.1 m) 1098 (66%)  847 (51%) 1094(65%) Example 8 CsNO₃ (<0.1 m) 1084 (65%)  824 (49%) 1089 (65%) Example9 NH₄NO₃ (0.013 m) 1117 (67%)  907 (54%) 1122 (67%) Example 10 Guanidinenitrate (0.02 m) 1026 (61%)  870 (52%) 1009 (60%) Example 11 KNO₂ (<0.1m) 1067 (64%)  833 (50%) 1073 (64%) Example 12 3,4-dinitro-toluene (0.4m) 1051 (63%)  846 (51%) 1026 (61%) Example 13 Nitromethane (0.4 m) 1107(66%) 1023 (61%) 1128 (67%) Example 14 Nitromethane (0.02 m)  996 (59%) 855 (51%)  992 (59%) Example 15 LiNO₃ (0.4 m) 1065 (64%)  968 (58%)1106 (66%) Example 17 1-nitropropane (0.4 m) 1053 (63%)  949 (57%) 1052(63%) Example 18 TEMPO (0.09 m)  919 (55%)  793 (47%)  907 (54%) Example19 1-ethyl-3-methyl 1186 (71%)  904 (54%) 1171 (70%) imidazolium nitrate(0.12 m)

Cells were stored for 24 hours at 25° C. before the 6^(th) dischargecycle. TABLE 2 Charge-Discharge Efficiency Additive (Concentration,Charge  Discharge Example molal, m) (mAh) (mAh)  D₅/C₄ Comp. Ex. 1 None1400  928 66.3% Example 1 LiNO₃ (0.002 m) 1393 1097 78.8% Example 2LiNO₃ (0.1 m) 1345 1263 93.9% Example 3 LiNO₃ (0.2 m) 1282 1232 96.1%Example 4 LiNO₃ (0.4 m) 1204 1189 98.8% Example 5 LiNO₃ (1.55 m) 11681139 97.6% Example 6 LiNO₃ (0.4 m) 1200 1163 96.9% Example 7 KNO₃ (<0.1m) 1242 1131 91.0% Example 8 CsNO₃ (<0.1 m) 1276 1117 87.5% Example 9NH₄NO₃ (0.013 m) 1386 1150 83.0% Example 10 Guanidine nitrate 1400 105775.5% (0.02 m) Example 11 KNO₂ (<0.1 m) 1273 1099 86.3% Example 123,4-dinitro-toluene 1163 1083 93.1% (0.4 m) Example 13 Nitromethane (0.4m) 1226 1140 93.0% Example 14 Nitromethane (0.02 m) 1400 1026 73.3%Example 15 LiNO₃ (0.4 m) 1150 1097 95.4% Example 17 1-nitropropane (0.4m) 1156 1085 93.9% Example 18 TEMPO (0.09 m) 1400  947 67.6% Example 191-ethyl-3-methyl 1296 1222 94.3% imidazolium nitrate (0.12 m)

Comparative Example 2

The electrolyte was that of Comparative Example 1 except that lithiumthiocyanate (LiSCN) at a concentration of 0.25 m was incorporated in a0.5 m electrolyte solution of lithium imide in DOL. Cycling of the cellwas performed by the procedure of Comparative Example 1 with the resultsshown in Table 3. Charge-discharge cycles were continued until thedischarge capacity diminished to 900 mAh (874 mAh/g of sulfur; 52%utilization), which was 18 cycles and the accumulated capacity 17.4 Ah.

Example 20

The electrolyte was that of Comparative Example 2 except that lithiumnitrate at a concentration of 0.25 m (17.3 mg/g of solvent) wasincorporated in the electrolyte solution. Cycling of the cell wasperformed by the procedure of Comparative Example 1 with the resultsshown in Table 3. Charge-discharge cycles were continued until thedischarge capacity diminished to 900 mAh (874 mAh/g of sulfur; 52%utilization), which was 56 cycles and the accumulated capacity 63 Ah.

Example 21

The electrolyte was that of Comparative Example 2 except that lithiumnitrate at a concentration of 0.45 m (31.1 mg/g of solvent) wasincorporated in the electrolyte solution. Cycling of the cell wasperformed by the procedure of Comparative Example 1 with the resultsshown in Table 3. Charge-discharge cycles were continued until thedischarge capacity diminished to 900 mAh (874 mAh/g of sulfur; 52%utilization), which was 52 cycles and the accumulated capacity 57.4 Ah.

Example 22

The electrolyte was that of Example 20 except that the electrolytesolvent was a 80/20 mixture by weight of DOL and DME in place of DOL.Cycling of the cell was performed by the procedure of ComparativeExample 1 with the results shown in Table 3. Charge-discharge cycleswere continued until the discharge capacity diminished to 900 mAh (874mAh/g; 52% utilization), which was 37 cycles and the accumulatedcapacity 40 Ah.

Example 23

The electrolyte was that of Example 21 except that the electrolytesolvent was a 80/20 mixture by weight of DOL and DME in place of DOL.Cycling of the cell was performed by the procedure of ComparativeExample 1 with the results shown in Table 3. Charge-discharge cycleswere continued until the discharge capacity diminished to 900 mAh (874mAh/g of sulfur; 52% utilization), which was 63 cycles and theaccumulated capacity 68.6 Ah. TABLE 3 Additive Charge- (Concentration,Electrolyte Sulfur Discharge Example molal, m) Solvent UtilizationEfficiency Comp. Ex. 2 None DOL 54.5%   80% Example 20 LiNO₃ (0.25 m)DOL 65.8% 96.4% Example 21 LiNO₃ (0.45 m) DOL 66.6% 97.6% Example 22LiNO₃ (0.25 m) DOL/DME 66.7% 97.7% Example 23 LiNO₃ (0.45 m) DOL/DME66.7% 97.8%

Example 24

The electrolyte was that of Comparative Example 1 except that theelectrolyte solution was a 3.6 m solution of lithium nitrate in adioxolane/triglyme mixture (50/50 by weight). Discharge-charge cyclingon the cell was performed at 50 mA/50 mA, respectively, with dischargecutoff at a voltage of 1.2 V and charge cutoff at 2.5 V. The dischargecapacity at the 5^(th) cycle was 1725 mAh and specific capacity 1675mAh/g of sulfur representing a utilization of 100%.

Example 25

The electrolyte was that of Comparative Example 1 except that theelectrolyte solution was a 2.7 m solution of lithium nitrate in adioxolane/diglyme mixture (50/50 by weight). Discharge-charge cycling onthe cell was performed at 50 mA/50 mA, respectively, with dischargecutoff at a voltage of 1.2 V and charge cutoff at 2.5 V. The dischargecapacity at the 5^(th) cycle was 1520 mAh and specific capacity 1485mAh/g of sulfur representing a utilization of 88.7%.

Example 26

The electrolyte was that of Comparative Example 1 except that theelectrolyte solution was a 1.5 m solution of lithium nitrate in adioxolane/dimethoxyethane/triglyme mixture (50/25/25 by weight).Discharge-charge cycling on the cell was performed at 350 mA/200 mA,respectively, with discharge cutoff at a voltage of 1.8 V and chargecutoff at 2.5 V. The discharge capacity at the 5^(th) cycle was 1316 mAhand specific capacity 1278 mAh/g of sulfur representing a utilization of76.3%.

Example 27

The electrolyte was that of Comparative Example 1 except that lithiumnitrate at a concentration of 0.75 m was incorporated in a 0.75 msolution of lithium imide in a 50/50 mixture of DOL and DME aselectrolyte. By the method of Comparative Example 1 fivedischarge-charge cycles were performed at 350 mA/200 mA, respectively,with discharge cutoff at a voltage of 1.8 V and charge cutoff at 2.5 V.Subsequent charge cycles were performed at a charge current of 200 mA toa 2.5 V cutoff. Discharge cycles 6 to 13 were carried out increasingdischarge currents from 100 mA (cycle 6) to 8400 mA (cycle 13) astabulated in Table 4. Sulfur utilization remained high, in excess of65%, even at the very high discharge rate of 3600 mA (3.6 A), a currentdensity of 4.26 mA/cm².

Example 28

The electrolyte was that of Comparative Example 1 except that lithiumnitrate at a concentration of 0.75 m was incorporated in a 0.75 msolution of lithium imide in a 50/50 mixture of DOL and DME aselectrolyte. By the method of Comparative Example 1 fivedischarge-charge cycles were performed at 350 mA/200 mA, respectively,with discharge cutoff at a voltage of 1.8 V and charge cutoff at 2.5 V.In the 6^(th) charge cycle at a charge current of 200 mA the voltage wasmeasured every 5 minutes, and change in voltage with time, dV/dt, involts/minute was calculated for each 5 minute interval. The value ofdV/dt until the cell voltage reached about 2.4 V was less than 0.002V/min. As the cell reached about 2.5 V the value of dV/dt increased to0.017 V/min and above 2.5 V dV/dt increased more to about 0.18 V/min.TABLE 4 Specific Capacity and Utilization vs. Discharge CurrentDischarge Specific Discharge Specific Current Discharge Current DensityCapacity Sulfur (mA) Current (mA/g) (mA/cm²) (mAh/g) Utilization 100 970.12 1302 77.7% 350 340 0.41 1218 72.7% 600 583 0.71 1141 68.1% 12001165 1.42 1184 70.7% 2400 2330 2.84 1126 67.2% 3600 3495 4.26 1107 66.1%6000 5825 7.09 854 51.0% 8400 8155 9.93 702 41.9%

While the invention has been described in detail and with to specificand general embodiments thereof, it will be apparent to one skilled inthe art that various changes and modifications can be made thereinwithout departing form the spirit and scope of the invention. Hence, theinvention is not limited to the embodiments disclosed herein but isinstead set forth in the following claims and legal equivalents thereof.

1. A method of charging a lithium-sulfur electrochemical cell whereinthe method comprises: (A) supplying electrical energy to the cell at aconstant current; (B) monitoring the voltage during charging; and (C)terminating the charge when the monitored voltage is in the range ofabout 2.35 volts to about 2.7 volts; and wherein the cell comprises: (a)a cathode comprising an electroactive sulfur-containing material; (b) ananode comprising lithium; and (c) a nonaqueous electrolyte, wherein theelectrolyte comprises: (i) one or more nonaqueous solvents selected fromthe group consisting of acyclic ethers, cyclic ethers, polyethers, andsulfones; and (ii) one or more N—O additives.
 2. The method of claim 1wherein the electrolyte further comprises one or more lithium salts. 3.The method of claim 1 wherein the termination voltage is the range ofabout 2.4 volts to about 2.6 volts.
 4. The method of claim 1 wherein thetermination voltage is about 2.5 volts.
 5. A method of charging alithium-sulfur electrochemical cell wherein the method comprises: (A)supplying electrical energy to the cell at a constant current; (B)monitoring the voltage during charging; (C) supplying electrical energyuntil the monitored voltage reaches about 2.5 volts; (D) holding thecell voltage at about 2.5 volts while monitoring the current density;and (E) terminating the charge when the charge current density is lessthan 0.025 mA/cm²; and wherein the cell comprises: (a) a cathodecomprising an electroactive sulfur-containing material; (b) an anodecomprising lithium; and (c) a nonaqueous electrolyte, wherein theelectrolyte comprises: (i) one or more nonaqueous solvents selected fromthe group consisting of acyclic ethers, cyclic ethers, polyethers, andsulfones; and (ii) one or more N—O additives.
 6. The method of claim 5wherein the electrolyte further comprises one or more lithium salts. 7.The method claim 5 wherein the charge is terminated when the chargecurrent density is less than 0.012 mA/cm².
 8. The method of claim 1wherein the one or more N—O additives is selected from one or more ofthe group consisting of inorganic nitrates, organic nitrates, inorganicnitrites, and organic nitro compounds.
 9. The method of claim 8 whereinthe inorganic nitrate is selected from one or more of the groupconsisting of lithium nitrate, potassium nitrate, cesium nitrate, bariumnitrate, and ammonium nitrate.
 10. The method of claim 8 wherein theinorganic nitrite is selected from one or more of the group consistingof lithium nitrite, potassium nitrite, cesium nitrite, and ammoniumnitrite.
 11. The method of claim 8 wherein the organic nitro compound isselected from one or more of the group consisting of nitromethane,nitropropane, nitrobenzene, dinitrobenzene, nitrotoluene,dinitrotoluene, and nitropyridine.
 12. The method of claim 2 wherein theone or more lithium salts is selected from one or more of the groupconsisting of LiSCN, LiCF₃SO₃, and LiN(CF₃SO₂)₂.
 13. The method of claim1 wherein the concentration of the one or more N—O additives in theelectrolyte is from about 0.1 m to 2.0 m.
 14. The method of claim 1wherein the concentration of the one or more N—O additives in theelectrolyte is from about 0.1 m to 1.0 m.
 15. The method of claim 1wherein the concentration of the one or more N—O additives in theelectrolyte is from about 0.1 m to 0.5 m.
 16. The method of claim 2wherein the concentration of the one or more lithium salts in theelectrolyte is from about 0.2 m to about 2.0 m.
 17. The method of claim1 wherein the electroactive sulfur-containing material compriseselemental sulfur.
 18. The method of claim 1 wherein the anode compriseslithium metal.
 19. A method of charging a lithium-sulfur electrochemicalcell wherein the method comprises: (A) supplying electrical energy tothe cell at a constant current; (B) monitoring the voltage duringcharging; (C) calculating the rate of change of voltage with time(dV/dt); and (D) terminating the charge when the value of dV/dtincreases by more than 5 times; and wherein the cell comprises: (a) acathode comprising an electroactive sulfur-containing material; (b) ananode comprising lithium; and (c) a nonaqueous electrolyte, wherein theelectrolyte comprises: (i) one or more nonaqueous solvents selected fromthe group consisting of acyclic ethers, cyclic ethers, polyethers, andsulfones; (ii) one or more lithium salts; and (iii) one or more N—Oadditives.
 20. The method of claim 19 wherein the charge is terminatedwhen the value of dV/dt increases by more than 10 times.